WO2024235413A1

WO2024235413A1 - Reducing stray shaft currents in an electric machine

Info

Publication number: WO2024235413A1
Application number: PCT/DK2024/050116
Authority: WO
Inventors: Peter Mongeau
Original assignee: Vestas Wind Systems A/S
Priority date: 2023-05-17
Filing date: 2024-05-15
Publication date: 2024-11-21

Abstract

Systems and methods for reducing stray currents in a rotary shaft (34, 46) of an electric machine (26). A common mode choke (70) including a high-permeability material is placed proximate to the rotary shaft (34, 46) of the electric machine (26). The common mode choke (70) is positioned between a shaft support (50) that supports the rotary shaft (34, 46) and a rotor (40) of the electric machine (26), and increases the impedance of the rotary shaft (34, 46) to stray currents conducted through the rotary shaft (34, 46). This increase in impedance impedes the flow of stray currents in the rotary shaft (34, 46), thereby reducing stray currents in the shaft supports (50) and other drivetrain components.

Description

REDUCING STRAY SHAFT CURRENTS IN AN ELECTRIC MACHINE

Technical Field

This invention relates generally to electric machines, and more particularly to systems and methods for reducing stray currents in the rotary shaft of a generator of a wind turbine.

Background

Wind turbines produce electrical energy by converting the kinetic energy of wind into mechanical energy, and subsequently converting this mechanical energy into electrical energy. Referring now to Fig. 1 , a wind turbine 10 typically includes a rotor 12 that is operatively coupled to a generator 14 by a gearbox 16. The rotor 10 includes one or more blades 18 configured to capture the wind energy, and converts the captured wind energy into mechanical energy in the form of rotation. The generator 14 then converts the mechanical energy received from the rotor 12 into electrical energy. To optimize the capture of wind energy over a broad range of wind speeds, most large wind turbines allow the rotor 12 to rotate at a speed that varies with wind speed. However, because this causes the generator 14 to rotate at a variable speed, the frequency of the electricity generated by the generator 14 also varies with wind speed. To allow electrical power generated by the generator 14 to be provided to the grid 20, variable speed wind turbines normally include a power converter 22 that converts the variable frequency electrical power produced by the generator to power having a fixed frequency (e.g., 50 or 60 Hz) and voltage compatible with the grid 20. One common type of power converter 22 includes a machine side inverter (not shown) that converts the output of the generator to DC electricity, and a grid-side inverter (not shown) which converts the DC electricity to AC electricity having the grid frequency and voltage, and operatively couples the machine side inverter to the grid 20.

Electric machines, such as generators, are often subject to stray currents in their rotary shafts, such as shaft-to-ground and shaft end-to-end currents. Shaft end-to-end currents are typically induced by internal inductive coupling, while shaft-to-ground currents are more commonly caused by capacitive coupling of voltages to the rotary shaft. For example, the voltage inverters used in the wind turbine power converter 22 typically generate high frequency switching noise that is then capacitively coupled into the rotary shaft of the generator 14.

Generator rotary shafts are typically supported by one or more bearings that include rolling elements. These rolling elements are normally located by bearing races and lubricated with a non-conductive material. In operation, a “bearing capacitor” may form across each bearing as the rolling elements ride around in the races and a dielectric film of lubricant forms between the bearing elements. During operation of the generator 14, stray currents in the rotary shaft may cause voltages across the bearing and/or other mechanical components connected to the shaft (e.g., gears, planet carriers, etc.) to exceed the breakdown voltage of the lubricating film. When this occurs, the stray currents may arc across the components in question, causing pitting and increasing mechanical wear. Stray currents can thereby shorten the life of mechanical components coupled to the rotary shaft, and increase the maintenance costs of the wind turbine 10.

Accordingly, there is a need for systems and methods that reduce stray currents in the rotary shafts of electric machines. More particularly, there is a need to reduce stray currents in the rotary shafts of generators in wind turbines that are connected to the grid by power converters.

Summary

In an aspect of the invention, an electric machine is provided. The electric machine is operatively coupled to a power converter, and includes a stator, a rotor configured to rotate relative to the stator, a rotary shaft operatively coupled to the rotor and supported by a shaft support, and a common mode choke proximate to the rotary shaft. The common mode choke is positioned along the rotary shaft between the shaft support and the rotor, and increases the impedance of the rotary shaft to reduce stray currents through the rotary shaft originating from the power converter.

In one embodiment of the electric machine, the common mode choke may include a toroid having an aperture through which the rotary shaft passes. In one embodiment, for example, the toroid may be continuous. The toroid may also be made from one or more of a ferrite, an iron-based alloy, or an amorphous metal alloy.

In one embodiment of the electric machine, the common mode choke may define a magnetic circuit that encircles the rotary shaft. The common mode choke may also include a conductive element magnetically coupled to the common mode choke such that a current flowing through the conductive element induces a magnetic field in the common mode choke.

In one embodiment of the electric machine, the electric machine may further include a damper circuit operatively coupled to the conductive element. The damper circuit may include a reactive element that increases an electrical impedance of the rotary shaft at a predetermined frequency. In one embodiment, for example, the predetermined frequency may be a peak frequency of a stray current in the rotary shaft. In another embodiment of the electric machine, the electric machine may include a current sensor configured to measure a stray current in the rotary shaft. The damper circuit may then be operatively coupled to the current sensor, and configured to output a voltage to the conductive element that reduces the stray current measured by the current sensor.

In another embodiment of the electric machine, the electric machine may further include a transmission configured to operatively couple the rotary shaft to a prime mover/mechanical load, and the shaft support may be provided by the transmission.

In another aspect of the invention, a wind turbine is provided that includes any of the embodiments of the electric machine described above.

In still another aspect of the invention, a method of suppressing stray current in the rotary shaft of the electric machine operatively coupled to the power converter and including the stator is provided. The method includes positioning the common mode choke along the rotary shaft between the shaft support that supports the rotary shaft and the rotor of the electric machine so that the common mode choke increases the impedance of the rotary shaft to reduce stray currents through the rotary shaft originating from the power converter.

In one embodiment of the method, the common mode choke may include the toroid having the aperture, and the rotary shaft may pass through the aperture.

In another embodiment, the common mode choke may include the conductive element magnetically coupled to the common mode choke such that the current flowing through the conductive element induces the magnetic field in the common mode choke, and the method may further include coupling an impedance to the conductive element that increases the impedance of the rotary shaft, and/or measuring the stray current in the rotary shaft and injecting an opposing current into the conductive element that reduces the stray current measurement.

Brief Description of the Drawings

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate various embodiments of the invention and, together with the general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the embodiments of the invention.

Fig. 1 is a schematic view of a wind turbine. Fig. 2 is a cross-sectional diagrammatic view of an electro-mechanical subassembly of an electric machine having a rotary shaft and a common mode choke that increases an impedance of the rotary shaft.

Figs. 3 and 4 are cross-sectional diagrammatic views of a portion of the electro-mechanical assembly of Fig. 2 showing additional details of the common mode choke.

Figs. 5 and 6 are perspective views of alternative embodiments of the common mode chokes of Figs. 2-4.

Figs. 7 and 8 are cross-sectional diagrammatic views of an alternative electro-mechanical assembly including stray current suppression systems having common mode chokes and damper circuits that are operatively coupled to the common mode chokes.

Fig. 9 is a schematic view of a passive damper circuit that may be used with the stray current suppression systems of Figs. 7 and 8.

Fig. 10 is a graphical view of stray current verses time for an exemplary electro-mechanical system of a wind turbine.

Figs. 11 and 12 are schematic views of active damper circuits that may be used with the stray current suppression systems of Figs. 7 and 8.

Figs. 13 and 14 are schematic views of power coupling circuits that may be used to provide power to the damping circuits of Figs. 11 and 12.

Detailed Description

Embodiments of the invention are directed to systems and methods for reducing stray electric currents in the rotary shafts, shaft bearings, and other mechanical components (such as gears) coupled to the rotary shafts of electric machines. The impedance of the rotary shaft is increased by placing a common mode choke proximate to and along the rotary shaft in an area where stray shaft currents are to be reduced. As used herein, “proximate to the rotary shaft” means that the radial distance between the common mode choke and the rotary shaft is small enough to produce an increase in the impedance of the rotary shaft that materially decreases (e.g., by 10% or more) a stray shaft current in the rotary shaft as compared to when the common mode choke is not present. Because the meaning of proximate scales with the size of the electric machines and common mode chokes, as well as the frequency of the stray current being suppressed, a common mode choke may be proximate to a rotary shaft if it is a millimeter or less from the shaft in a small electric machine (e.g., a 1 kW generator), a centimeter or less from the shaft in a medium sized electric machine (e.g., a 100 kW generator), and a decimeter or less from the shaft in a large electric machine (a 10 MW generator). Because the impedance change provided by the common mode choke can be limited by its cross-section, it may be advantageous to minimize the radial distance between the common mode choke and rotary shaft so as to maximize the available cross-section of the common mode choke within the space available. Thus, the minimum distance may depend on the available space, the tolerances of the hardware (e.g., runout of the rotary shaft, axial alignment between the common mode choke and shaft, etc.), and the need to service the electric machine.

To increase the impedance of the rotary shaft, the common mode choke includes a high-permeability material that interacts with magnetic fields produced by stray currents to impede the flow of the stray currents through the rotary shaft. As used herein, high-permeability materials refer to materials such as ferrite, iron-based alloys (e.g., permalloy), nanocrystalline iron-based alloys, amorphous metal alloys, alloy including nickel and/or cobalt, or any other material having a relative permeability / ₀ greater than 100.

Fig. 2 depicts a cross-sectional view of an exemplary electro-mechanical assembly 24 including an electric machine 26 (e.g., a generator), a transmission 28 (e.g., a planetary gearbox), and a prime mover/mechanical load 30 (e.g., the main shaft housing of a wind turbine rotor) that is supported by a bedplate 31 or other support structure. The transmission 28 includes a housing 32 and a transmission shaft 34, e.g., a high-speed shaft. The electric machine 26 includes a housing 36, a stator 38 having an armature 39, and a rotor 40. The rotor 40 includes a field magnet assembly 42, and a rotor hub 44. The rotor hub 44 includes a rotor shaft 46 and a flange 48 that operatively couples the rotor shaft 46 to the field magnet assembly 42. The rotor shaft 46 is operatively coupled to the transmission shaft 34, and is supported by one or more shaft supports 50, e.g., two shaft supports 50. The shaft supports 50 enable the transmission and rotor shafts to rotate with respect to the housing 36 of electric machine 26, and locate the rotor 40 within the stator 38. Although the shaft supports 50 are depicted as bearings, the shaft supports 50 may also be provided by other support structures, such as gears, planet carriers, or other suitable components configured to support the rotor shaft 46 and/or transmission shaft 34.

The housing 32 of transmission 28 may be rigidly coupled to the housing 36 of electric machine 26 so that the electric machine 26 and transmission 28 are maintained in a fixed relationship. The transmission shaft 34 is configured to couple rotation between the transmission 28 and the rotor 40 of electric machine 26 through the rotor shaft 46. The transmission shaft 34 and rotor shaft 46 thus operate cooperatively to transmit rotation between the transmission 28 and rotor 40, and may be referred to individually or collectively herein as a rotary shaft. In some embodiments, the electric machine 26 may be directly connected to the transmission 28 proximate to the shaft supports 50, and/or the shaft supports 50 may be located in the transmission 28. The rotor shaft 46 may also include or be coupled directly to one or more mechanical components of the transmission 28 (e.g., a gear and/or planet carrier), in which case these mechanical components may include or otherwise provide the one or more shaft supports 50.

The stator 38 may be operatively coupled to the housing 36 of electric machine 26 by one or more electrically insulating couplers 52 so that the stator 38 is galvanically isolated from the housing 36. The rotor 40 may be capacitively coupled to the stator 38 via an air gap 54 between the field magnet assembly 42 of rotor 40 and the armature 39 of stator 38. Due to the galvanic isolation of the stator 38 and the floating nature of the rotor 40, the primary path for conductive coupling between the generator 14 and the shaft supports 50 in the electro-mechanical assembly 24 may be through the rotor shaft 46.

The stator 38 and rotor 40 may have a concentric arrangement, with the stator 38 being fixed and stationary, and the rotor 40 being rotatable relative to the stator 38. As the rotor 40 rotates, it produces a rotating magnetic field that interacts with the stator 38 to produce electrical power. Although the exemplary electric machine 26 is depicted as having only one rotor 40 and one stator 38 with a concentric arrangement, it should be understood that embodiments of the invention may be used with electric machines 26 having one or more rotors 40 or one or more stators 38. These electric machines 26 may also include a stator 38 disposed radially inside a rotor 40, or include rotors 40 and stators 38 having an axial flux topology. Although the electric machines described herein have the field magnet assembly 42 as part of the rotor 40 and the armature 39 as part of the stator 38, it should be understood that embodiments of invention may also be used with electric machines having stationary magnetic assemblies and rotating armatures. It should also be understood that the electric machines described herein may operate as either a generator that converts rotational energy into electrical energy (generating mode), or as a motor that converts electrical energy into rotational energy (motoring mode).

The armature 39 of stator 38 may be electrically coupled to a power converter 56 including a machine-side inverter 58 that converts the output of the stator 38 to direct current (DC) electricity and which is operatively coupled to a grid-side inverter 60. The grid-side inverter 60 operatively couples the machine-side inverter 58 to the grid 20, and converts the DC electricity to alternating current (AC) electricity having the grid frequency and voltage. The bedplate 31 , stator 38, and power converter 56 may each be electrically coupled to a common ground point 62 by a separate ground return 64-66 to reduce coupling of stray currents from the machine-side inverter 58 into the electro-mechanical assembly 24 through the stator 38. The bedplate 31 may also be separately electrically coupled to Earth ground, e.g., through a tower of a wind turbine of which the electro-mechanical assembly 24 is a part. One or more of the ground returns 64-66 (e.g., the ground return 66 of bedplate 31) may include some amount of parasitic inductance 68 due to its length.

Figs. 3 and 4 depict detailed views of a portion of the electro-mechanical assembly 24 of Fig. 2 illustrating placement of a common mode choke 70 along the rotor shaft 46 between the rotor 40 of the electric machine 26 and the shaft supports 50. The common mode choke 70 may be made from a high-permeability material and include an aperture 72 through which the rotor shaft 46 passes. The common mode choke 70 may be mechanically coupled to and rotate with the rotor shaft 46 (i.e., the common mode choke 70 is in a rotating frame - Fig. 3), or there may be a gap 74 between an outer surface 76 of the rotor shaft 46 and an inner surface 78 of the common mode choke 70 (i.e., the common mode choke 70 is in a stationary frame - Fig. 4). The gap 74 may allow the common mode choke 70 to remain stationary while the rotor shaft 46 rotates, in which case the common mode choke 70 can be held in place, for example, by mechanically coupling the common mode choke 70 to the housing 36 of electric machine 26. As described in more detail below, the common mode choke 70 serves to resist any transient net current that tries to flow through any shaft around which the common mode choke 70 is placed.

When a stray current /(f) flows through a rotary shaft (such as the transmission shaft 34 or rotor shaft 46), the current generates a magnetic field B(t) that circulates around the shaft. This magnetic field B(t) has a magnitude that depends on the magnitude of the stray current /(t). Any change in the magnitude of the stray current /(f) results in a corresponding change in the magnitude of the magnetic field B(t). However, any change in the magnitude of the magnetic field B(f) induces a corresponding electromotive force in the rotary shaft that opposes the change in the stray current /(t). This tendency for an electrical conductor to oppose changes in the flow of electricity through it is referred to as “inductance”. Inductance is defined as the ratio of the voltage induced in the conductor to the rate of change of the current flowing through the conductor. Because this ratio depends at least in part on the permeability /J of materials proximate to the conductor, the inductance of one or both of the transmission shaft 34 and rotor shaft 46 can be increased by adding the common mode choke 70 due to the effects of the high- permeability material from which the common mode choke 70 is made. This inductance increases the electrical impedance encountered by stray currents in the rotary shaft, thus reducing the magnitude of the stray currents.

Typically, the dominant electrical path between the stator 38 of electric machine 26 and the transmission 28 includes the capacitive coupling across the air gap 54 between the stator 38 and the rotor 40. This path is completed by conduction between the rotor 40 and transmission 28 through the rotary shafts. Thus, increasing the electrical impedance of the transmission shaft 34 and/or rotor shaft 46 may reduce stray currents in the rotational shafts connecting the electric machine 26 to the transmission 28. The net effect of adding the common mode choke 70 is to increase the electrical impedance between the rotor 40 and transmission 28, thereby reducing the amount of current injected into the transmission 28. Constricting this electrical path between the rotor 40 of electric machine 26 and the transmission 28 may thereby reduce stray currents passing through all drivetrain components downstream from the common mode choke 70, such as bearings and gears. Preferably, the common mode choke 70 should be located along the rotor shaft 46 so that the common mode choke 70 is in the electrical conduction path connecting the shaft supports 50 and the air gap 54.

Figs. 5 and 6 depict exemplary common mode chokes 70 each including a toroid 80 made of a high-permeability material. The common mode choke 70 depicted by Fig. 6 also includes a winding 82 formed by wrapping a conductive element around the toroid 80 to define a helical coil. The winding 82 includes a plurality of terminals 84 (e.g., two terminals), and is configured to output a voltage at the terminals 84 in response to the presence of stray currents flowing through the aperture 72 of common mode choke 70. The winding 82 may also induce a current in a conductor (e.g., shaft) passing through the aperture 72 in response to a current flowing through the winding 82, e.g., due application of an external voltage across the terminals 84. Although the exemplary toroid 80 is depicted as being continuous, embodiments may include high-permeability members having U-shapes or gaps that allow the common mode choke 70 to be placed over an installed rotary shaft, or that include multiple parts (e.g., two halves of a toroid) which can be coupled together around the rotary shaft to provide a magnetic circuit that encircles the rotary shaft.

Figs. 7 and 8 depict exemplary stray current suppression systems that include a common mode choke 70 and a damper circuit 86. The stray current suppression system of Fig. 8 further includes a current sensor 88 (e.g., a Rogowski coil) configured to detect currents flowing through the transmission shaft 34 (not shown) and/or rotor shaft 46. In each system, the damper circuit 86 is operatively coupled to a winding 82 of the common mode choke 70. In the system depicted by Fig. 8, the damper circuit 86 is also operatively coupled to the current sensor 88. In each case, the damper circuit 86 may be in the rotating frame or the stationary frame depending on whether the common mode choke 70 is rotating or stationary.

Fig. 9 depicts an exemplary passive damper circuit 86 which may include one or more reactive elements (e.g., a capacitor 90 and/or inductor 92) and/or a resistor 94. By way of example, when present, the capacitor 90 may have a value selected to form a resonant circuit with the winding 82 of common mode choke 70. The capacitor 90 may thereby increase the electrical impedance encountered by stray currents flowing through the aperture 72 of common mode choke 70 at a resonant frequency near that of the resonant circuit. The capacitor 90 may be used alone in this capacity, with the inductor 92 (e.g., to adjust the resonant frequency and/or quality factor of the resonant circuit), and/or with the resistor 94 (e.g., to reduce ringing and broaden the bandwidth of the resonant circuit).

Fig. 10 depicts an exemplary graph of shaft current versus time for a large (e.g., 15 MW) wind turbine. As can be seen from the graph, the shaft current includes a ringing component having a peak frequency content at about 200 kHz and a peak current level of about 3 amps. The frequency content of the stray current may be related to the frequency content of the switching noise generated by the power converter, and thus consistent over time. Because the root mean square current (l_RMs) is much lower than the peak current, damping the peak current may significantly reduce overall shaft current values. Thus, a passive damper circuit 86 configured to resonate at a predetermined frequency of about 200 kHz could significantly dampen the peak shaft current in this example.

Fig. 11 depicts an exemplary active damper circuit 86 including a negative impedance converter 98 that may be used to inject an opposing current into the common mode choke 70. The negative impedance converter 98 may include an operational amplifier 100, an impedance Z, and a pair of gain-setting resistors F?i, F?2. In response to current /(f) from the common mode choke 70, the negative impedance converter 98 may produce a voltage v(t) characteristic of a “negative impedance” in accordance with the following equation:

The active damper circuit 86 of Fig. 11 may thereby respond to the presence of a stray current passing through the common mode choke 70 by providing an opposing voltage which impedes the flow of the stray current. The impedance Z may include one or more resistors, capacitors, and inductors configured to generate a negative impedance Z_m optimized to impede the flow of stray current in the rotary shaft around which the common mode choke 70 is located.

Fig. 12 depicts an exemplary active damper circuit 86 that includes a proportional-integral- derivative (PID) control loop. This active damper circuit may be used in stray current suppression systems that include a current sensor 88, such as depicted by Fig. 8. An electrical signal generated by the current sensor 88 may be operatively coupled to a differential amplifier 108 to produce a proportional error signal 110 at the output thereof. The proportional error signal 110 may be provided to one or more of an integrator 114 that generates an integral error signal 111 and a differentiator 116 that generates a derivative error signal 112. Each of the proportional, integral, and derivative error signals 110-112 may then be provided to a respective proportional gain block 120, integral gain block 121 , and derivative gain block 122. The output signal from each gain block 120-122 may then be summed by a summation block 126 to produce a control signal 128. This control signal 128 may be provided to an amplifier 130 (e.g., a transconductance amplifier) that outputs an opposing current 132 proportional to the control signal 128. This opposing current 132 may be provided to the winding 82 of common mode choke 70 to oppose the stray current passing therethrough. The electromagnetic coupling between the common mode choke 70 and current sensor 88 may close the control loop, and allow the active damper circuit 86 to null-out stray currents in the shaft passing through the common mode choke 70 by driving the proportional error signal 110 towards zero.

Figs. 13 and 14 depict exemplary power coupling circuits that may be used to provide power to an active damper circuit 86. Fig. 13 depicts a direct current power coupler that includes series inductors 134, 136 and a filter capacitor 138 to reduce noise in the damper circuit 86. Fig. 14 depicts an alternating current power coupling that galvanically isolates the damper circuit 86. The alternating current power coupling includes a transformer 140 that couples an AC signal from an external power source 142 to a rectifier 144, which may be located on the same circuit board as the damper circuit 86. A filter capacitor 146 may be operatively coupled to the output of the rectifier 144 to reduce noise on the output thereof. Active damper circuits 86 in the rotating frame may have power coupled from an external source inductively or capacitively, or may include an internal power source, e.g., that harvests power from the rotation of the damper circuit 86.

While all the invention has been illustrated by a description of various embodiments, and while these embodiments have been described in considerable detail, it is not the intention of the Applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the Applicant’s general inventive concept.

Claims

1. An electric machine (26) operatively coupled to a power converter (56), comprising: a stator (38); a rotor (40) configured to rotate relative to the stator (38); a rotary shaft (34, 46) operatively coupled to the rotor (40) and supported by a shaft support (50); and a common mode choke (70) proximate to the rotary shaft (34, 46) and positioned along the rotary shaft (34, 46) between the shaft support (50) and the rotor (40), wherein the common mode choke (70) increases the impedance of the rotary shaft (34, 46) to reduce stray currents through the rotary shaft (34, 46) originating from the power converter (56).

2. The electric machine (26) of claim 1 , wherein the common mode choke (70) includes a toroid (80) having an aperture (72), and the rotary shaft (34, 46) passes through the aperture (72).

3. The electric machine (26) of claim 1 or 2, wherein the toroid (80) is continuous.

4. The electric machine (26) of claim 2 or 3, wherein the toroid (80) is made from one or more of a ferrite, an iron-based alloy, or an amorphous metal alloy.

5. The electric machine (26) of any of the preceding claims, wherein the common mode choke (70) defines a magnetic circuit that encircles the rotary shaft (34, 46).

6. The electric machine (26) of any of the preceding claims, further comprising: a conductive element (82) magnetically coupled to the common mode choke (70) such that a current flowing through the conductive element (82) induces a magnetic field in the common mode choke (70).

7. The electric machine (26) of claim 6, further comprising: a damper circuit (86) operatively coupled to the conductive element (82).

8. The electric machine (26) of claim 7, wherein the damper circuit (86) includes a reactive element (90, 92) that increases an electrical impedance of the rotary shaft (34, 46) at a predetermined frequency.

9. The electric machine (26) of claim 8, wherein the predetermined frequency is a peak frequency of a stray current in the rotary shaft (34, 46).

10. The electric machine (26) of any of claims 7-9, further comprising: a current sensor (88) configured to measure a stray current in the rotary shaft (34, 46), wherein the damper circuit (86) is operatively coupled to the current sensor (88) and configured to output a voltage to the conductive element (82) that reduces the stray current measured by the current sensor (88).

11 . The electric machine (26) of any of the preceding claims, further comprising: a transmission (28) configured to operatively couple the rotary shaft (34, 46) to a prime mover/mechanical load (30), wherein the shaft support (50) is provided by the transmission (28).

12. A wind turbine, comprising: the electric machine (26) of any of claims 1-10.

13. A method of suppressing a stray current in a rotary shaft (34, 46) of an electric machine (26) operatively coupled to a power converter (56) and including a stator (38), comprising: positioning a common mode choke (70) along the rotary shaft (34, 46) between a shaft support (50) that supports the rotary shaft (34, 46) and a rotor (40) of the electric machine (26), wherein the common mode choke (70) increases the impedance of the rotary shaft (34, 46) to reduce stray currents through the rotary shaft (34, 46) originating from the power converter (56).

14. The method of claim 13, wherein the common mode choke (70) includes a toroid (80) having an aperture (72), and the rotary shaft (34, 46) passes through the aperture (72).

15. The method of claim 13 or 14, wherein the common mode choke (70) includes a conductive element (82) magnetically coupled to the common mode choke (70) such that a current flowing through the conductive element (82) induces a magnetic field in the common mode choke (70), and further comprising: coupling an impedance to the conductive element (82) that increases an electrical impedance of the rotary shaft (34, 46); and/or measuring the stray current in the rotary shaft (34, 46), and injecting an opposing current into the conductive element (82) that reduces the stray current measurement.